1. Field of the Invention
The present invention relates to a linear induction motor for an elevator and, more specifically, to a linear induction motor for an elevator which is used as an apparatus for driving the elevator.
2. Description of the Related Art
Hitherto, elevators employing linear induction motors as driving apparatuses have been disclosed, for instance, in Japanese Patent Laid-Open No. 57-121568.
FIG. 5 shows the structure of a conventional linear induction motor for an elevator which is of the flat-element two-sided induction type, and which is the same as that shown in "Linear Motors and Their Application" (pages 14 to 27; published by Japanese Electrotechnical Committee (JEC) in March 1984). Referring to FIG. 5, a secondary, stationary element 1 made of aluminum and having a thickness of t is provided in an elevator shaft (not shown) in such a manner as to vertically extend. A primary, movable element 2 comprising primary iron cores 2a and windings (not shown) wound thereon is provided on a counter-weight (not shown) which is vertically movable along the secondary, stationary element 1. The movable element 2 has two mutually opposing portions between which portions the stationary element 1 is positioned. Although in FIG. 5, these portions of the movable element 2 are shown as separate parts on either side of the stationary element 1, they are in fact parts of a single member that are integral with each other. Gaps 3 and 4, each having a dimension of g, are defined between two opposing surfaces of the secondary, stationary element 1 and the two portions of the primary, movable element 2.
In the conventional linear induction motor for an elevator which has the above-described construction, when alternating current is supplied to the windings of the primary, movable element 2, magnetic flux, such as the flux 5 indicated by the arrows, is generated by the corkscrew rule. The magnetic flux 5 moves progressively. On the other hand, the magnetic flux 5 causes eddy currents, such as the currents 6 shown in FIG. 6, to flow in the secondary, stationary element 1. The magnetic flux 5 and the eddy currents 6 together allow a thrust to be produced in accordance with Fleming's rule, whereby the primary, movable element 2 is driven.
At this time, the dimension of the total magnetic gap in the linear induction motor corresponds to the result obtained by adding, to the sum of the respective dimensions g of the gaps 3 and 4, the thickness t of the secondary, stationary element 1, in other words, 2g+t.
The above-described construction of the conventional linear induction motor for an elevator entails the following problem. Since the secondary, stationary element 1 whose length is determined by the length of elevator shaft can be considerably long, it is difficult to keep the element 1 straight throughout the length thereof with a high level of precision. On the other hand, if the dimension g of the gaps 3 and 4 are extremely small, there is a risk that the primary movable element 2 may contact the secondary, stationary element 1. In order to avoid this risk, the dimension g of the gaps 3 and 4 cannot be reduced to an extreme degree. For this reason, it has been difficult to reduce the dimension of the total magnetic gap in the linear induction motor. Furthermore, since the secondary, stationary member 1 consists of a single conductor made of aluminum, this construction inevitably involves ineffective eddy currents, such as the currents 7 indicated by the broken lines in FIG. 6, which flow in the stationary element 1 in vain because these currents do not serve to produce thrust. Because of the total magnetic gap that cannot easily be reduced and because of the ineffective eddy currents, the conventional linear induction motor for an elevator suffers from a problem in which the driving efficiency cannot be substantially increased.